Combustion inhibitor for gas-producing charges



United States PatetitO COMBUSTION INHIBITOR FOR GAS-PRODUCING CHARGESJames N. Bohn, Kenvil, N. 1., and Allan G. Sandholf,

Cumberland, Md., assignors to Hercules Powder Company, Wilmington, DeL,a corporation of Delaware No Drawing. Application May 25, 1953 SerialNo. 357,384

Claims. (Cl. 26030.6)

This invention relates to gas-producing charges having surfaces whichare inhibited or restricted from burning and to plastic inhibitorcompositions for this purpose.

In the contemporary development of solid propellants, it has been foundnecessary to the attainment of desirable characteristics to inhibit somesurfaces of the combustible gas-producing charges from burning. Up tothe present time the most desirable inhibitor has been found to becellulose acetate. Cellulose acetate has been-applied to grains ofgas-producing compositions and in particular to rocket grains ofdouble-base smokeless powder in many ways. Initially the grains wereinhibited by cementing plastic patches to the surface or applying aconvolute wrapping of cellulose acetate sheet material suitablymoistened with anadhesion promoting agent such as acetone or by suitableadhesive composition. As the art developed, the inhibitor structureswere preformed and, in accordance with one procedure, the gas-producingcharge is cast directly into preformed cylindrical cellulose acetatetubes. In accordance with alternative methods, the charge is initiallyprepared by casting or extrusion and then fitted into a hollow cylinderof cellulose acetate which had been prepared by wrapping or extrusion.Restriction of end surfaces has usually been achieved by bondingpreformed plates to the desired portions of the grain ends.

The cellulose acetate inhibitor and inhibitors made of similar materialssuch as ethyl cellulose have been reasonably satisfactory and havecontributed greatly to the development of improved gas-producingcharges. However, it has been recognized for several years that even thepreferred cellulose acetate restriction left much to be desired.Probably one of the most serious faults of cellulose acetate is found inthe fact that when the usual double-base smokeless powder grain isrestricted with this plastic material, an undesirably large amount ofnitroglycerin migrates from the powder grain into the inhibitor duringstorage. Of probably equal seriousness is the migration of plasticizerfrom the inhibitor layer into the base grains. Ballistically the twoeffects are additive, since loss of nitroglycerin from the grain andpick-up of cooling plasticizer by the grain lower the overall poten-'tial. This problem has not been a serious one where the powder grainswere employed immediately or where the time of storage was short.However, it is evident that for military uses in particular, it isusually necessary to stockpile large numbers of such gas-producinggrains under varying conditions of storage for long periods of time. Foraccuracy and optimum performance it is essential that the grains performin substantially the same manner at the end of a storage period as atthe beginning of the storage period. Depending upon the amount ofnitroglycerin migration into the inhibitor, the characteristics of thegrain necessarily change during storage. Due to the leaching of thenitroglycerin from the peripheral surface of the grain, the ballisticsofthe charge are changed cellulose acetate inhibitor, there has been acontinuous r eifort on the part of the explosives art to develop a ma"and the grain structure ,itself becorries characterized by p 2,8582%Patented Oct. 28, s.

2 diiferent structural strengths and ignition characteristics dependingupon the nitroglycerin content at a particular point. Furthermore,burning time is seriously affected by plasticizer migration. It isapparent that where the gas producing charge is employed as the solemeans of propulsion of a guided missile such variations in the makeupand structural characteristics of the burning chargecan have seriousramifications with respect to accuracy' forced to thicker inhibitors,this is an undesirable ap--- proach to the problem, since for the samesize rocket;

motor and a given propellant composition any increase: in the thicknessof the inhibitor layer must necessarily work a proportional decrease inthe amount of gas-producing .pay load.

In view of these recognized faults of even the preferredl terial for useas a restriction which will give the neces sary bond between theinhibitor and the gas-producing:

charge, which will not absorb nitroglycerin or other ex. plosiveplasticizer in any substantial amount, and which,

will retain the necessary bonded relationship for the nec-- essaryperiod of time even though subjected to varying: storage conditions interms of temperature changes andl the like. This effort on the part ofthe art has been completely unsuccessful, however, and at the presenttime cel! lulose acetate, despite its known faults, is still consideredto be the best available combustion inhibitor for most gas-producingcharges.

Now in accordance with the present invention, plastic compositions maybe prepared and gas-producing charges: may be fabricated employing suchplastic compositions; which are characterized by greatly improvedproperties..

The compositions of the invention will form excellent? bonds withgas-producing charges, particularly those con-- taining nitrocellulose,without detrimental migration of? explosive plasticizer from thegas-producing grain.

Generally described, the present invention is a gasproducing chargecomprising a body of combustible gasproducing composition having atleast part of its surface:

covered by a burning inhibitor layer of plastic composition bondedthereto, said plastic composition comprising the cured product of amixture containing from 25 to 50% monomeric butyl methacrylate, from 25to 45%= of a finely-divided copolymer of vinyl chloride with not. morethan about 5% of vinyl acetate, from 5 to 30% of finely-dividedpolymeric methyl methacrylate, and 5 to 20% of a nonvolatile plasticizerfor the vinyl chloride-- vinyl acetate copolymer. The gas-producingcharge is: preferably a smokeless powder composition and may bea singlebase formulation comprising nitrocellulose, but? will ordinarily be adouble-base composition containingv nitrocellulose and nitroglycerin orsimilar explosive plas-- ticizer such as other liquid explosive nitricesters. Thegas-producing charge may also be formed from othercombustible materials currently employed by the art' such as ammoniumnitrate and nitroguanidine which may be compacted into suitable physicalform with or without the use of a binding agent or matrix suchasnitrocellulose or various synthetic resins; Thegas-producing charge mayhave any desired configuration but in accordance withcurrent practiceisnormally a cylindrical grain having its outer cylindrical surfaceinhibited from burning in accordance with the invention. An inhibitorlayer may also be employed on either or both ends of the grain dependingupon the internal geometry of the grain. In some embodiments, such as inperforated grains, it is desirable to inhibit the outer surface andperforation surface and allow only end burning.

Also included in the invention are the cured and uncured'inhibitorcompositions comprising from 25 to 50% monomeric butyl methacrylate,from 25 to 45% of a copolymer of vinyl chloride with not more than about5% of vinyl acetate, from 5 to 30% of polymeric methyl methacrylate, and5 to 20% of a nonvolatile plasticizer for the vinyl chloride-vinylacetate copolymer. In accordance with the invention and in order toobtain even stronger bonds which will survive repeated cycling from lowto high temperature in storage, amounts up to and including 15% of abifunctional cross-linking monomer may also be included in thecomposition. The most desirable bifunctional cross-linking monomer isthe dimethacrylate of polyethylene glycol 200 (BCM). Improved resultshave also been obtained with divinyl benzene, although some bondfailures after extended storage at 66% C. have still been encountered.With BCM, on the'other hand, excellent results are still obtained atsuch temperatures.

As will be illustrated in the subsequently presented examples, it hasbeen found that the ranges of incorporation for the various constituentsof the plastic inhibiting material of this invention are critical toobtaining a satisfactory inhibitor, particularly when the inhibitor isemployed to restrict from burning surfaces of the usually employedsmokeless powder charges comprising nitrocellulose and nitroglycerin.

The use of less than 25 parts of butyl methacrylate normally results inbond failure during storage, while the use of more than 50 parts ofbutyl methacrylate monomer results in a mixture which allows thepolymeric constituents of the composition, and particularly thepolymethyl methacrylate, to settle out prior to curing. Furthermore,when a composition containing more than 50 parts of butyl methacrylatemonomer is employed, the resulting cured product becomes brittle withresulting bond failure and cracking of the restriction during curing,storage, and temperature cycling.

If less than 25 parts of polyvinyl chloride'acetate copolymer isemployed, bond failure normally results and nitroglycerin is absorbedfrom a double-base gas-producing charge of smokeless powder. On theother hand, if more than 45 parts of the copolymer'are employed, theuncured mix rapidly becomes too viscous to handle satisfactorily andalso prevents the formation of even a satisfactory initialpowder-inhibitor bond. In addition, the increased viscosity makes itprohibitively difiicult to remove entrapped air from the uncuredcomposition. Furthermore, it has been found that if the copolymercontains more than about 5% vinyl acetate, either the composition doesnot set up at all or else the bond fails even 4 at room temperaturestorage due to the softness of the composition.

The presence of at least 5% polymethyl methacrylate is essential to theformation of a satisfactory bond. However, if more than 30% ofpolymethyl methacrylate is employed, bond failures are again obtainedduring storage.

The presence of at least 5% of a plasticizer for the vinylchloride-vinyl acetate copolymer is essential to a composition whichdoes not become so brittle upon curing that the bond between the powdergrain and the inhibitor is destroyed. However, if more than 20 parts ofplasticizer is employed, the resistance of the inhibitor tonitroglycerin absorption is again lowered and failure of the inhibitoritself is 'usually obtained in storage.

The incorporation of the bifunctional cross-linking monomer is optionaldepending upon the use to which the restricted gas-producing charges areto be put. It is preferred, however, that the cross-linking agent beemployed in all compositions since its presence has been found topreserve the integrity of the bond between the gas-producing charge andthe inhibitor under all temperature conditions which can reasonably beexpected even during military operations. The cross-linking obtained bythe incorporation of the cross-linking agent does cause a hard ening ofthe inhibitor. However, the amount of hardening obtained is notdisadvantageous as long as not more than 15 parts of the cross-linkingagent is employed. When more than 15 parts is'employed, undesirablebrittleness of the cured product is obtained with resulting bondfaliures.

The inhibitor compositions in accordance with the invention cure rapidlywith conventional polymerization catalysts for acrylic compounds and atlow temperatures. Cure time depends on the thickness of the restrictionlayer and the type and amount of catalyst.

Having generally described the invention, the following examples aregiven in order to illustrate operable compositions and gas-producingcharges and to illustrate the criticality of the proportions of theplastic inhibitor composition of the invention.

In Table 1, Examples l31 are included to demonstrate operable inhibitorcompositions and to illustrate the necessity of observing thelimitations above set forth. Each of the compositions set forth wasapplied to a cylindrical grain of double-base smokeless powdercontaining nitroglycerin by concentrically disposing the powder grain inan oversized cylindrical mold equipped with a steam jacket and fillingthe resulting annulus between grain and mold with the uncured mix. Thecomposition was cured for about one hour at a temperature of C. Afterremoval from the mold, the restriction and bond were examined. Theinhibited grains were then placed in hot storage and unless otherwiseindicated, were maintained at about 60 C. for a period of three weeksand examined at weekly intervals. forth in-Table -1 under Remarks. 'Allthe compositions in Table 1 contained about 4% of Luperco ATC (a 50:50mixture of benzoyl peroxide and tricresyl phos phate) as apolymerization catalyst.

TABLE 1 Plasticizer Percent Cross- Remarks linking Agent good bond-goodplastic. bond failed in hot storage-plastic softened. good bondgoodplastic. satisfactory bondplastic satisfactory but soigencd somewhatafter hot storage.

good bond-good plastic. good bondgood plasticuncured mix thin withslight settling out of PMMA. bontlijfailed in hot storageplasticsoftened. 0.

good bond-plastic satisfactory but somewhat brittle. bond failed inhotstorage-plastic cracked.

The'observations made are set i TABLE, IQ-Co'ntinued BMA PMMAPlastieizer Percent Crossg Agent Remarks 25 (15% DAP) 15(10% TCP).

b) M03 one. 8

co can cc c5 so 7 9 to woo woo o co o o M HM m an O0 8o 8 88 8 C we: coc: we:

In Table 2, Examples 32-37 are presented to illustrate the criticalityof the 95/5 ratio of PVC/PVA. Thirty (30) percent of the PVC-PVAcopolymer was employed 5 DVB (18%)--- 10 DVB (18%) 1o DVB 45% 5 DvB 45%10 BCM in 2111 cases. 1 The commercial designation of the polymer 40employed is set forth in parenthesis. The inhibitor was applied todouble-base smokeless powder grains by the bond good (in hot storage theplastic softened undesirably). good bond-good plastic. satisfactorybondplastic satisfactory but softened somewhat in hot storage. goodbond-plastic satisfactory but exhibited slight softening in hot storage.bond failed in hot storageplastic softened. satisfactory bond-plasticsatisfactory but softened slightly. 1 bond failed in hot storage-plasticsatisfactory. satisfactory bond-plastic satisfactory but becomingbrittleuncured mix thin and some PMMA settled out while standing. bondfailed in hot storageplastic satisfactory -uncured mix was very thinallowing the PMMA to settle out. good bond-good plastic.

satisfactory bondsatisfactory plasticuncured mix viscous and difiicultto handle. satisfactory bond-plastic satisfactory but showed somesoftening in storage. satisfactory bond-satisfactory plastic-uncured mixviscous and difficult to handle. bond failedplastic very brittle andcracked. good bondgood plastic-excellent temperature properties. goodinitial bond-good plastic-bond failed after extended storage at 66 0.good initial bond-plastic satisfactory although somewhat brittle. goodbondgood plastic. good initial bond-good plastic-bond failed afterextended storage at 66 0. good bond-good plasticexcellent temperatureproperties.

outer surfaces of the remaining groups were restricted in accordancewith the invention. The outer surfaces of group 2 were restricted bycasting around the grains a mix containing 40% monomeric butylmethacrylate,

polyvinyl chloride-acetate copolymer, 20% polymethyl methacrylate, and10% of tricresyl phosphate. Two percasting process employed in Examples1-31. Observacent of benzoyl peroxide based on the weight of butyl tionsare given under Remarks. methacrylate monomer was employed aspolymerization TABLE 2 Exlaimple BMA PMMA PL PVC/PVA Catalyst* Remarks32 40 20 10 TOP.-- 95/5 (VYNV-l) p 4 excellent for all uses exceptextended hot storage. 33 40 20 10 TGP 8588/12-15 (VYHH) 4 very thick,very tacky, air

bubblesno casting. 34 40 2O 10 TOP-.- 100/-(QYNV) 4 Bond failed-becomesvery g softinhot storage. 35 4o 20 10 TOP.-- ss.5-91.5/s.5-11.5 4 verythick-air bubbles-no (VYNS-3) casting. 1 36. 20' 10 TCP 100/ (Geon 121)4 bond failure. 37 40 20 10 SEER... 100l-(Geon 121) 4 Do.

7 BMiX-n -butyl methacrylate monomer. PMMA-polymethyl methacrylate. F66r iiii i 1 111 d t t 1 -p0 yvmy c on e ace a e copo ymer. TCP-tricresylphosphate. SHP-di-Z-ethylhexyl tetrahydrophthalate.

Catalyst-Luperco ATO (/50 benzoyl peroxide) tricrcsyl phosphate.

"Even when the-amount of PVA in the copolymer is 5% or less, difiicultyis sometimes encountered in obtaining a casting if the particle size ofthe copolymer is unduly large The copolymers which are commerciallyproduced for plastisols and organosols and contain less than 5% of PVAare generally satisfactory as furnished. Any difi'iculty encountered canbe overcome by allowing -more time for the copolymer to go into solutionor by ball-milling the PVC-A to reduce particle size before .use.

Example 38 Four groups of solid, cylindrical, end-burning grains wereprepared. The outer surfaces of group 1 were con- 60 C. storage for 36weeks and at the end of each week a grain from each group was removedand its restriction analyzed to determine nitroglycerin absorption. Inthe analysis the standard TiCl method on a methylene chloride extractwas employed. At the end of 12 weeks the cellulose acetate restrictioncontained 29% by weight ventionally restricted with cellulose acetate,while the of nitroglycerin. At the end of 12 weeks it was noted thatnitroglycerin absorption had practically leveled off in all therestrictions. At the end of 36 weeks, the restrictions of groups 2, 3,and 4 contained a maximum of 8%, and 7.5%, respectively.

Example 39 Restricted grains similar to those of groups 2, 3, and 4 wereplaced in 80 C. storage for 9 weeks. Bond failures occured in group 2 atthe end of two days. At the end of 9 weeks plastic and bonds in groups 3and 4 were oxide is especially preferred in view of the low temperaturenecessary to effect polymerization with this material.

. In some applications it may be desired to incorporate a fillermaterial in the restriction composition such as glass wool, asbestosfioc, and the like.

Since modifications of the invention will be apparent to the art, theinvention is limited only by the scope of excellent. Maximum absorptionof nitroglycerin in group I 3 was 10.7% and in group 4 was 9.8%.

The plastic inhibitor compositions of the invention may be applied toexternal or perforation surfaces of gasproducing charges in any desiredmanner. The preferred method is to place an unrestricted grain into anoversized mold and then introduce the curable mix or slurry into theannulus around the grain in such a manner as to prevent entrapment ofair. This is best accomplished by introducing the mix under pressureinto the base of the mold. The entire mold is then subjected to curingconditions until the plastic has set and a bond has been formed.Perforation surfaces may be similarly inhibited by employing undersizedforms and then introducing the uncured mix into the space between formand perforation surface. Alternatively, the plastic inhibitor may beinitially molded and the propellant grain subsequently cast in thehollow inhibitor cylinder, thus using this cylinder as a mold or castingcontainer. A satisfactory bond is obtained between inhibitor and grainduring the curing of the grain. In other instances it may be desired toapply the composition by crosshead extrusion techniques. Anotheroperable method, although not as desirable, is to separatelyprefabricate both the inhibitor and the grain to close tolerances, applya mutual solvent to the surface of the grain, and then slide the graininto the inhibitor cylinder.

It is especially preferred, however, to cast the inhibitor around thefinished grain or in a perforation, since it is by this procedure thatthe best bond is obtained. The enhanced bond thus produced results fromthe solvent action of the monomeric butyl methacrylate monomer on thenitrocellulose on the surface of the gas-producing grain. Since thebutyl methacrylate is also a solvent for the polymeric constituents ofthe inhibitor, the nitrocellulose polymer and the polyvinyl chlorideacetatepolymethyl methacrylate of the inhibitor are held in mutualsolution at the surface of the grain. Thus, when the butyl methacrylateis polymerized, a strong bond is obtained. The strength of this bond andthe ruggedness of the inhibitor composition as a whole is furtherenhanced by cross-linking of the butyl methacrylate when thebifunctional cross-linking monomer such as BCM or divinylbenzene isemployed.

It has been found that any of the conventional plasticizers for PVC-Aemployed by the art are operable when employed in the critical amount.Of all of those employed, however, tricresyl phosphate is preferred inview of its plasticizing properties and further in view of itsfireproofing effect on the composition. Other plasticizers which may beemployed include, without limitation, diallyl phthalate, trioctylphthalate, trioctyl phosphate, dibutyl sebacate, and dibutyl adipate.

A variety of polymerization catalysts for acrylic monomers are known tothe art and may be employed in place of any of the catalysts illustratedby the examples. Such materials include lauroyl peroxide, di-tert-butylperoxide, tertiary butyl hydroperoxide, cumene hydroperoxide, methylethyl ketone peroxide, and tertiary butyl perbenzoate. Of all of theoperable catalysts those of the peroxide type are the most desirable.Benzoyl perthe appended claims.

What we claim and desire to protect by Letters Patent V 1. A curablemixture adapted for inhibitor application to combustible gas-producingcharges comprising by weight from 25 to 50% monomeric butylmethacrylate, from 25 to 45% of a copolymer of vinyl chloride with notmore than about 5% of vinyl acetate, from 5 to 30% of polymeric methylmethacrylate, and from 5 to 20% of a nonvolatile plasticizer for thevinyl chloride-vinyl acetate copolymer.

, 2. A mixture according to claim 1 containing a polymerization catalystfor monomeric butyl methacrylate.

3. A mixture according to claim 1 in which the plasticizer is tricresylphosphate. 4. A curable mixture adapted for inhibitor application tocombustible gas-producing charges comprising by weight from 25to 50%monomeric butyl methacrylate, from 25 to 45 of a copolymer of vinylchloride with not more than about 5% of vinyl acetate, from 5 t6 30% ofpolymeric methyl methacrylate, an amount not exceeding 15% of thedimethacrylate of ethylene glycol,

and from 5 to 20% of a nonvolatile plasticizer for the vinylchloride-vinyl acetate copolymer.

5. A mixture according to claim 4 containing a polymerization catalystfor monomeric butyl methacrylate.

6. A mixture according to claim 4 in whichthe'p'lasticizer is tricresylphosphate.

7. A composition of matter comprising the cured product of a mixturecontaining from 25 to 50% monomeric butyl methacrylate, from 25 to 45 ofa copolymer of vinyl chloride with not more than about 5% of vinylacetate, from 5 to 30% of polymeric methyl methacrylate, and from 5 to20% of a nonvolatile plasticizer for-the vinyl chloride-vinyl acetatecopolymer, said product being adapted for inhibitor application to acombustible gasproducing charge.

8. A composition according to claim 7 in which the plasticizer istricresyl phosphate.

9. A composition of matter comprising the cured product of a mixturecontaining from 25 to 50% monom'er ic butyl methacrylate, from 25 to 45of a copolymer of vinyl chloride with not more than about 5% of vinylacetate, from 5 to 30% of polymeric methyl methacrylate, an amount notexceeding 15 of the dimethacrylate of ethylene glycol, and from 5 to 20%of a nonvolatile plasticizer for the vinyl chloride-vinyl acetatecopolymer, said product being adapted for inhibitor application to acombustible gas-producing charge.

10. A composition according to claim 9 in which the plasticizer istricresyl phosphate.

References Cited in the file of this patent UNITED STATES PATENTS2,155,591 Garvey Apr. 25, 1939 2,392,857 McGill Jan. 15, 1946 2,476,993Milton et al July 26, 1949 2,498,388 Ball Feb. 21, 1950 2,643,184 CairnsJune 23,1953 2,657,191 Coover et a1. Oct. 27, 1953 2,666,042 Nozaki"Jan. 12, 1954 FOREIGN PATENTS 540,383 Great Britain Oct. 15, 1941

1. A CURABLE MIXTURE ADAPTED FOR INHIBITOR APPLICATION TO COMBUSTIBLEGAS-PRODUCING CHARGES COMPRISING BY WEIGHT FROM 25 TO 50% MONOMERICBUTYL METHACRYLATE, FROM 25 TO 45% OF A COPOLYMER OF VINYL CHLORIDE WITHNOT MORE THAN ABOUT 5% OF VINYL ACATATE, FROM 5 TO 30% OF POLYMERICMETHYL METHACRYLATE, AND FROM 5 TO 20% OF A NONVOLATILE PLASTICIZER FORTHE VINYL CHLORIDE-VINYL ACETATE COPOLYMER.